1. Field of the Invention
The present invention relates to a device assembly structure, and in particular, to an assembly structure appropriate for assembling the outer leads of a tape carrier package device onto a circuit board.
2. Description of the Prior Art
In recent years, in order to cope with the reduction in size and weight of electronic equipments, semiconductor devices have been developed to have a high function and a large capacity. In accordance with this trend, semiconductor devices have been made to have a large size and an increased number of terminals, and a variety of methods for mounting such a device on a circuit board are under examination.
For example, an outer lead bonding method using a tape carrier package device (referred to as a TCP device hereinafter) has been put into practical use for the reason that the method is capable of mounting multi-terminal large-sized devices at a high density.
FIG. 17 is a sectional view showing a thinnest assembly structure for a prior art TCP device.
In FIG. 17, reference numeral 1 generally denotes a TCP device, and this TCP device 1 is provided with a semiconductor chip 11 covered with a protecting resin 15, leads 12 connected to the semiconductor chip 11 via a bump 13a and a support ring 14 for supporting the leads 12.
The leads 12 of the TCP device 1 are electrically and mechanically connected with electrodes 22 on a circuit board 20 by solder 31 with the semiconductor chip 11 received in a device hole 21 formed in the circuit board 20. In the figure, reference numerals 16 and 23 denote solder resists.
Generally, a portion 12a of the lead 12 which extends inwardly of the support ring 14 to be connected to the semiconductor chip 11 is referred to as an inner lead, while a portion 12b of the lead 12 which extends outwardly to be connected to the circuit board 20 is referred to as an outer lead.
In the assembly structure of this prior art, when the device 1 is mounted, the outer leads 12b are in a substantially straight state without being formed. By placing the semiconductor chip 11 in the device hole 21 formed in the circuit board 20, the device 1 does not protrude much above the circuit board 20, thereby allowing a thin assembly structure to be achieved.
In the prior art assembly structure in which no lead forming is performed, the outer leads 12b are extended straightly. This, however, has caused a problem that, when a temperature change occurs during a heat applying stage in the mounting process or during operation of the device which causes generation of heat, the leads 12 tend to be broken by a thermal stress attributed to a difference in coefficient of thermal expansion between the TCP device 1 and the circuit board 20. That is, the constituent members have their respective inherent coefficients of thermal expansion, and according to a change of environmental temperature, the members expand or contract at different coefficients of thermal expansion.
For example, in the assembly structure shown in FIG. 17, assuming that the coefficient of thermal expansion of the circuit board 20 is greater than the coefficient of thermal expansion of the TCP device 1, the circuit board 20 tends to expand more when the environmental temperature is increased. Consequently, in the TCP device 1, a tensional stress is generated in a direction in which the leads 12 extend. This stress causes a distortion in the TCP device 1. As shown in FIG. 18 which is an enlarged view of a section B in FIG. 17, the distortion occurs most frequently in a weakest portion of the outer lead 12b on the chip center side in the vicinity of a connection portion of the outer lead 12b with the solder 31 (the portion indicated by an arrow C) where the lead 12 is not reinforced by the support ring 14 nor by the protecting resin 15. Therefore, a metal fatigue occurs in the leads 12 due to a change of environmental temperature, and eventually the leads 12 will be broken. Particularly when the leads are fine, or very thin (when the connection pitch is small), the lead breakage occurs frequently.
In order to solve such a problem, a method for alleviating the stress by bending the leads as disclosed in the Japanese Patent Laid-Open Publication No. HEI 4-199647 has been proposed.
However, this method requires a region for bending the leads, and this causes a reduction of mounting density. Moreover, since the length of the lead itself increases, the lead tend to be deformed, consequently causing a problem that the lead connection at a fine pitch can be hardly achieved.
To solve the problem, a structure as shown in FIG. 19 can be considered in which after the bonding process of the outer lead 12b of a TCP device 101, the portion of the outer lead 12b that is not reinforced by the support ring 14 nor by the protecting resin 15 is coated with an insulating resin 18 for the purpose of reinforcement of the outer lead 12b.
The above structure, however, has the following problems.
That is, in soldering the outer lead 12b to the electrode 22 of the circuit board 20, a flux is generally used in order to remove oxide substances on the connection surfaces of the electrode 22 and the lead 12 to thereby assure a good connection. However, in view of the recent issue of regulating fluorocarbon materials due to environmental problems and for easy handling, non-cleaning type fluxes are often used.
Then, such a non-cleaning type flux leaves its residues at the connection portion and its periphery even after the soldering is achieved. Therefore, when the insulating resin 18 is applied on the outer leads 12b, spaces between the outer leads 12b and 12b and spaces between the outer leads 12b and the circuit board 20 are filled with the insulating resin 18 with the interposition of the flux residues, so that each outer lead 12b is fixed relative to the adjacent outer lead 12b and the circuit board 20 with the presence of the flux residues. However, the flux is not made such that it has a function of retaining an adhesion of the resin to the board and a function of reinforcing the connection portion. Therefore, the adhesion surface is readily peeled off by the stress. Also the flux itself tends to be damaged. Therefore, a sufficient reinforcing effect cannot be obtained by the fixation of the outer leads 12b with the insulating resin 18.